Conically shaped anterior fusion cage and method of implantation

ABSTRACT

A fusion cage for promoting fusion with one or more bone structures including a conically-shaped cage body including a proximal end having a first diameter and a distal end having a second diameter, wherein the first diameter of the proximal end is greater than the second diameter of the distal end.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present invention is a continuation of application Ser. No.09/358,188, filed Jul. 20, 1999, which is a continuation of applicationSer. No. 08/781,525 filed Jan. 9, 1997, which is a file wrappercontinuation of application Ser. No. 08/306,879 filed Dec. 15, 1994 (nowabandoned). The disclosures of the '188, '525 and '879 applications arehereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] This invention is directed to devices and methods forfacilitating the fusing of bone structures and more particularly thefusing together of adjacent vertebral bodies or bone structures.

[0003] Technical literature and patent documents disclose a number ofdevices and methods for fusing bones together. One such device which hasproven to be successful is disclosed in U.S. Pat. No. 4,961,740,entitled “V-THREAD FUSION CAGE AND METHOD OF FUSING A BONE JOINT,” whichpatent has been assigned the present assignee and which patent isincorporated herein by reference. The referenced patent discloses afusion cage which is preferably cylindrical and has threads formed aspart of the external cylindrical surface. The fusion cage defines aninternal cavity and apertures through the wall of the cage whichcommunicate the external cylindrical surface with the internal cavity.The apertures are formed in the valleys of the threads. Normally twosuch cages are used to stabilized and fuse together adjacent vertebralbodies or bone structures.

[0004] In practice, using a posterior approach, a patient's vertebralbone structures are exposed and degenerate disk material located betweenthe vertebral bone structures is removed. A threaded tap is used to tapcomplementary threads in the upper and lower vertebral bone structurespreparatory to the insertion of the above fusion cage. Once such tappinghas been accomplished, using an introduction tool, the fusion cage isscrewed into the space between the adjacent vertebral bone structures.The threads bite into the bone of the upper and lower vertebral bonestructures, stabilizing the bone structures, and preventing the fusioncage from working out of this position due to patient movement.Generally two such fusion cages are applied using this technique. Oncethe two implants have been positioned, then bone growth inducingsubstances, such as bone chips, are packed into the internal cavityof-the fusion cages. These bone growth inducing substances come intoimmediate contact with the bone from the vertebral bone structures whichproject into the internal cavity through the apertures. Such projectionof bone is due to the fact that the apertures are formed in the valleysof the external threads of the fusion cage. Such immediate bone to bonecontact between the vertebral bone structures and the bone pack withinthe fusion cages results in more rapid propagation of bone cells betweenthe adjacent vertebral bone structures and thus a more rapid fusion ofthe adjacent vertebral bone structures.

SUMMARY OF THE INVENTION

[0005] The present invention is directed to a fusion cage which has beendesigned to be implanted using an anterior approach to the vertebralbone structures.

[0006] In a first embodiment of the present invention, the fusion cageincludes a conically-shaped cage body having a proximal end and a distalend, said distal end having a diameter which is smaller than thediameter of the proximal end. The distal end further is rounded with forexample a bull nose in order to facilitate the insertion of the cagebody relative to one or more bone structures. The conically-shaped cagebody is particularly advantageous for use with an anterior approach tovertebral bone structure fusion. This is due to the fact that the normallordosis of the vertebral bone structures defines a wedged-shape spacefor a vertebral disk between, for example, lumbar vertebrae.Accordingly, the conically-shaped body cage can be sized and selected inorder to maintain or enlarge upon the normal lordosis.

[0007] In a second embodiment of the present invention, a fusion cageincludes a conically-shaped cage body having a proximal end and a distalend with the distal end having a diameter which is smaller than thediameter of the proximal end. The conically-shaped cage body has aconically-shaped outer surface and at least one flute formed in theconically-shaped outer surface. The flute acts as a relief much as theflute placed on self-tapping screws in order to facilitate the insertionof the fusion cage using a twisting motion between two vertebral bonestructures.

[0008] In a third embodiment of the invention, a fusion cage includes aconically-shaped cage body having a proximal end and a distal end, thedistal end having a diameter which is smaller than the diameter of theproximal end. The conically-shaped cage body has a conically-shapedouter surface and a plurality of threads formed as part of theconically-shaped outer surface. The plurality of threads allows the cagebody to be inserted using an anterior approach. Due to the fact that thecage body is conically-shaped, the requirement for pretapping thevertebral bone structures to receive the fusion cage is eliminated withthe fusion cage being self-tapping. Also the cage gradually spreadsapart the vertebral bone structures as the cage is inserted in order toregain or enlarge the natural lordosis of the adjacent vertebral bonestructures. As with other embodiments of the present invention, flutescan be provided through the plurality of threads in order to allow forenhanced thread tapping by the cage and for a smoother insertion of thefusion cage between the vertebral bone structures. Preferably two orthree flutes would be formed spaced about the fusion cage in order thatone flute would be engaging with or adjacent to an upper vertebral bonestructures with another flute being engaging with or adjacent to a lowervertebral bone structure. Such a relationship maintains alignment of thefusion cage and prevent wandering as the fusion cage is introducedbetween the two vertebral bone structures. Without two or more flutes,wandering might occur due to the fact that the threads are onlysubstantially engaged with the vertebral bone structures and not withthe disk material between the vertebral bone structures, which diskmaterial does not provide support to the threads.

[0009] In a further aspect of the invention, any of the aboveembodiments can be provided with a plurality of apertures through thefusion cage and an internal cavity with the apertures communicatingbetween the internal cavity and the external surface of the fusion cage.Bone growth inducing substances, such as bone chips, can be packed intothe internal cavity either before the fusion cage is inserted or afterthe fusion cage has reached a final insertion position. The bone chipscome in contact with the vertebral bone structures through the aperturesin order to facilitate fusion between the adjacent vertebral bonestructures.

[0010] In another aspect of the invention which can be included in anyof the above embodiments, the cage body can have a round or bull nosedistal end with one or more flutes formed in the round or bull nosedistal end in order to enhance the self-tapping nature of the fusioncage.

[0011] In yet another aspect of the invention, introduction tools allowthe fusion cage to be accurately positioned between the vertebral bonestructures.

[0012] The method of the present invention affords access to adjacentvertebral bone structures using an anterior approach and procedure. Suchanterior approach and procedure can be preferably performedlaparoscopically using an introduction set including a cannula. Alaparoscopic procedure is minimally invasive as the abdomen muscletissue can be spread using a set of cannula of increasing size and asmall opening thereby developed through which a fusion cage can beinserted. Such a procedure is less traumatic to the tissue than analternate anterior approach and procedure, also known as an anteriorlumbar interbody fusion, where an incision, perhaps up to five incheslong is made, through the abdomen muscle tissue. It is to be understoodhowever that either anterior approach and procedure can be used with thefusion cage and fall within the scope of the invention.

[0013] After such access, using preferably a laparoscopic technique,degenerate disk material can be removed and, using a cannula andinsertion tool, an appropriately shaped fusion cage can be screwed intoplace between the vertebral bone structures in order to stabilize thevertebral bone structures and allow for fusion. Either preparatory toinsertion of the fusion cage or after it has been inserted, bone chipsor other bone growth inducing substances can be inserted into the fusioncage to promote bone to bone contact and subsequent fusion.

[0014] It is to be understood that although the above embodiments havebeen described with respect to the fusion of adjacent vertebral bodiesor bone structures, that the present invention can be used to fusetogether a variety of bone structures, in addition to being fused to onebone structure and used as, for example, a base for an implant.

[0015] Other objects and advantages of the invention can be obtainedthrough a review of the specification and the figures.

BRIEF DESCIPTION OF THE FIGURE

[0016]FIG. 1 is a partially sectional side view of an embodiment of thefusion cage of the invention.

[0017]FIG. 2 depicts a left end (distal end) view of the fusion cage ofFIG. 1.

[0018]FIG. 3 depicts a right end (proximal end) view of the fusion cageof FIG. 1.

[0019]FIG. 4 depicts a view through line 4-4 of the fusion cage of FIG.1.

[0020]FIG. 5 depicts fusion cage of FIG. 1 in conjunction with anintroduction tool.

[0021]FIG. 6 depicts an alternative embodiment of the introduction tool.

[0022]FIGS. 7, 8, and 9 depict progressive stages in the method ofinserting the fusion cage between adjacent vertebral bone structures.

[0023]FIG. 10 depicts a side view of an alternative embodiment of thefusion cage of the invention.

[0024]FIG. 11 depicts the left end (distal end) view of the fusion cageof FIG. 10.

[0025]FIG. 12 depicts the right end (proximal end) view of the fusioncage of FIG. 10.

[0026]FIG. 13 depicts a side view of yet another embodiment of thefusion cage of the present invention.

[0027]FIG. 14 depicts a left distal end (distal end) view of the fusioncage of the invention of FIG. 13.

[0028]FIG. 15 depicts a right end (proximal end) view of the fusion cageof the invention of FIG. 13.

[0029]FIG. 16 depicts a sectional view taken through line 16-16 of FIG.13.

DETAILED DESCRIPTION

[0030] With respect to the figures in a particular FIG. 1, a side viewof the preferred embodiment of the fusion cage 20 is depicted. Fusioncage 20 includes a fusion cage body 22 which in this preferredembodiment is provided in the shape of a cone. Fusion cage 20 includes adistal end 24 and a proximal end 26. The distal end 24 in a preferredembodiment is rounded or bull nosed in order to facilitate the insertionof the fusion cage 20 relative to one or more bone structures. Theproximal end 26 includes an opening 28 which communicates with aninternal cavity 30 defined by the fusion cage 20. The opening 28 in apreferred embodiment is threaded so that it can receive an end cap orplug 32 (FIG. 5). End cap 32 is used to close off the proximal end 26and retain bone growth inducing substances packed therein as describedhereinbelow. As can be seen in FIG. 5, end cap 32 includes a threadedbore 34 which is designed to receive an insertion tool. The threadedbore 34 has an initial unthreaded, hex-shaped section 35 which can beused with a socket wrench to tightly position end cap 32 in opening 28.The proximal end 26 further define first and second peripheralindentations 36, 38. These peripheral indentations 36, 38 receive tangsfrom an insertion tool as described hereinbelow for facilitating theinsertion of the fusion cage 20.

[0031] A plurality of threads 40 is defined as part of the outercylindrical surface 42 of the body 22. It is to be understood that theplurality of threads can be replaced with a plurality of interrupted ordiscrete threads or a plurality of projections, ridges, protrusions,barbs, or spurs and be within the spirit and scope of the invention.

[0032] The rounded distal end 24, and at least some of threads 40defined flutes or relief grooves 42, 44, and 46. (FIGS. 1, 2.) In apreferred embodiment, flutes 42, 44, and 46 meet at a central point 48of the distal end 24 on the longitudinal axis 50 of the fusion cage 20.In other embodiments the flutes can be smaller and not extend all theway to the central point 48 on the longitude axis 50. Still in otherembodiments, the flutes can be eliminated from the distal end 24 andsuch embodiments are still within the spirit and scope of theinvention.he flutes extend from the distal end 24 toward the proximalend 26 as shown in FIG. 1 with respect to flute 42. These flutes aredefined by the sections 52 which are removed from the threads. In apreferred embodiment, the flutes become narrower as they approach theproximal end 26 due to the fact that thread relief for purposes ofself-tapping becomes less important as the cage reaches a final restingposition. As shown in other embodiments, the flutes can be deeper andextend from the distal end completely to the proximal end. Still furtherin other embodiments the flutes can be confined to the first severalthreads adjacent to the distal end and/or to just the distal end.

[0033] As can be seen in FIGS. 1, 4, a plurality of apertures 54 areprovided through wall 56 of the fusion cage 20. In a preferredembodiment, these apertures 54 are formed by broaching grooves 58 in theinternal surface 60 of the internal cavity 30. The effect of suchbroaching is to remove material from the valleys between the threads 40,thus defining the aperture 54. The advantages of such an arrangement aretaught by the above-referenced U.S. Pat. No. 4,961,740, which patent isincorporated herein by reference and allows for immediate bone to bonecontact between the vertebral bodies or bone structures and the bonepacked within the internal cavity 30 of the fusion cage 20.

[0034] The apertures 54 in a preferred embodiment increase in size fromsmaller apertures closer to the distal end 24 to a larger aperturecloser to the proximal end 26. This increase in size allows for morebone to bone contact. Alternatively in the embodiment as shown in FIG.1, all the apertures are of the same size.

[0035] As can be seen in FIG. 4, the apertures are clustered about atransverse axis 51, both at the upper and lower end of the axis. This isso that in position, the apertures come into contact with the upper andlower vertebral bone structures (FIG. 9) to encourage bone growththrough the fusion cage from the vertebral bone structures. The lateralsection of the fusion cage found along the other transverse access 53 donot have apertures in order to prevent growth of disk material whichmight interfere with the bone fusing process.

[0036] A preferred embodiment of the conically-shaped fusion cage 20includes a fusion cage which is 23 millimeters in length having a distalend 24 with a diameter of 14 millimeters and a proximal end 26 with adiameter of 18 millimeters. The cage body is a right circular cone. Thethreads have a pitch of 300 and there are ten threads per inch with athread depth of 0.053 inches. Further the cage is made of a titaniummaterial. Preferably this and the other disclosed fusion cages disclosedare machined. However, the processes such as molding can be used toaccomplished formation of the fusion cages.

[0037] The cage is inserted between vertebral bodies using an insertiontool 62 (FIG. 5). Insertion tool 62 includes an inner handle 64 and anouter handle 66. The outer handle includes a bore 68 for receiving theinner handle 64. Handles 64, 66 include knobs 70, 72 respectively. Thedistal end of inner handle 64 defines a threaded shaft 74, having areverse thread to facilitate easy removal, and the distal end of handle66 define a cylindrical disk 76 which has first and second tangs 78, 80,projecting from the peripheral edge of the cylindrical disk 76. Thesetangs 78, 80 are designed to mate with the peripheral indentation 36, 38of the fusion cage 20. For purposes of inserting the fusion cage betweenthe vertebral bodies, the end cap 32 is inserted into the fusion cage 20as shown in FIG. 5. Then the threaded shaft 74 of the inner handle isintroduced into the threaded bore 34 of the end cap 32. After this isaccomplished, the outer handle 66 is slid over the inner handle 64 andthe tangs 78, 80 are positioned into engagement with the indentations36, 38. In this arrangement, the fusion cage 20 can be anteriorlyinserted into the space between the vertebral body structure using theinsertion tool 62.

[0038] An alternative embodiment of the insertion tool is shown in FIG.6. In this figure, insertion tool 82 includes a handle 84 with a knob86. At the end of the insertion tool 82 distal from the knob 86 is acylindrical disk 88 which has first and second tangs 90, 92, which havethe same function as the above tangs 78, 80. Extending from the centerof the cylindrical disk 88 along the centerline of the insertion tool 82is a shaft 94 which has a ball detent 96. For use with insertion tool82, the -threaded bore 34 of the end cap 32 would be replaced with abore having a lip which could engage with the ball detent 96 of theinsertion tool 82.

[0039] The method for inserting the fusion cage 20 of FIG. 1 using ananterior approach and procedure to the vertebral bodies is as follows.It is to be understood that although the focus of this discussion is ona laparoscopic procedure, that the anterior approach and procedure canalso include a more invasive procedure where a long incision is made inthe abdomen wall.

[0040] With an anterior approach, using an introduction set such asdescribed by way of example only, in U.S. Pat. No. 4,863,430, entitled“INTRODUCTION SET WITH FLEXIBLE TROCAR WITH CURVED CANNULA,” which isincorporated by reference, but however with larger diameter instruments,an amount of disk material is removed between the two vertebral bodiesor bone structures which are to be fused together. This procedure isaccomplished through a cannula position adjacent to the vertebral bonestructures. With the same or a larger diameter cannula, the fusion cage20 can be introduced adjacent to the vertebral bone structures. In afirst procedure, the fusion cage is packed with bone growth substancesand the end cap 32 is affixed to the fusion cage 20. Insertion tool 62is then secured to the fusion cage 20 and the fusion cage is guidedthrough the cannula to a location adjacent to the upper and lowervertebral body such as presented schematically in FIGS. 7, 8, 9, byupper body 98 and lower body 100. In the initial position as shown inFIG. 7, the fusion cage 20 is adjacent to the anterior surfaces 102, 104of the vertebral bodies 98, 100. As the introduction tool is turned, thethreads 40 of the fusion cage 20 bite into the vertebral bodies 98, 100.Further turning of the introduction tool causes the fusion cage to movethrough the position shown in FIG. 8 to the final resting position shownin FIG. 9, where the distal end 24 is moved adjacent to the posteriorsections 106, 108 of the vertebral bone structures 98, 100. As thisoccurs, the fusion cage 20 increases the lordosis or spacing between thevertebral bodies, basically distracting the vertebral bodies and causingthe vertebral bodies to pivot about the posterior sections 106, 108,with such posterior sections acting like a hinge. It is noted that mostof the distraction occurs adjacent to the anterior sections, but thatdistractions also occur at the posterior sections where the hingedeffect is exhibited. Preferably, the lordosis. is increased over thenormal lordosis in order to stabilize the vertebral bone structuresprior to fusion occurring. Stabilization occurs due to the fact thatincreased lordosis places additional stress on the anterior longitudinalligaments which are part of the anatomy holding the vertebral bodies inplace.

[0041] Once the fusion cage 20 is appropriately positioned, the handle64 of the insertion tool 62 is unscrewed from the cap 32 and the fusionhandle 62 is pulled away from the fusion cage.

[0042] An alternative embodiment of a fusion cage 200 is shown in FIGS.10, 11, and 12. Fusion cage 200 includes a distal end 202 and an aproximal end 204. Fusion cage 200 includes an internal cavity 206. Endcaps not shown can be used to close the ports 208, 210 of distal andproximal ends 202, 204. A plurality of threads 212 are defined on theexternal conical surface 214 of the fusion cage 200. Defined by thethreads 212 are first and second flutes 216, 218, which in thisembodiment extend from the distal end 202 to the proximal end 204. Theseflutes provide thread relief allowing the fusion cage 200 to beself-tapping.

[0043] The fusion cage 200 includes a plurality of elongated apertures220 which are formed through the side walls of a fusion cage 200. Theelongated apertures 202 are formed in such a way that the internalconical surface 214 is spaced away from the internal surface 224 of theinternal cavity 206 by the thickness of the sidewall 222.

[0044] A further embodiment of the invention is shown in FIGS. 13, 14,15 and 16. In FIG. 16 the fusion cage 300 has distal and proximal ends302 and 304 respectively. The fusion cage 300 defines an internal cavity306, and ports 308 and 310 defined through the distal and proximal ends302 and 304 respectfully. A plurality of thread 312 is defined as partof the external conical surface 314 of the fusion cage 200. First,second and third flutes 316, 318, and 320, are defined in the threads312 from the distal end 302 to the proximal end 304. These flutes givethe fusion cage 300 an enhanced self-tapping advantage. These flutes areequally spaced about the fusion cage 300 in a manner similar to theflutes of the fusion cage embodiment 20 in FIG. 1.

[0045] A plurality of aperture 322 is provided through the externalconical surface 314 of the fusion cage 300 and through the side wall 324opening into the internal cavity 306. Accordingly, at the location ofthe aperture 322 the external surface 314 is held away from the internalsurface 326 by the thickness of the side wall 324.

[0046] The present invention affords the advantages of a fusion cagewhich can be introduced through an anterior approach in order tomaintain or increase lordosis between adjacent vertebral bodies. Thefusion cage has the advantage of being conically-shaped and self-tappingthrough the use of external flutes. The flutes additionally assist inkeeping the fusion cage aligned and centered as the cage is beinginserted between the vertebral bone structures.

[0047] Other advantages, aspects, and objects of the invention can beobtained through a review of the claims and the appended figures.

[0048] It is to be understood that additional embodiments of theinvention can be constructed and fall within the spirit and scope of theclaims.

1. A fusion cage for promoting fusion with one or more bone structures comprising: a conically-shaped cage body including a proximal end having a first diameter and a distal end having a second diameter, wherein said first diameter of said proximal end is greater than said second diameter of said distal end.
 2. The fusion cage as claimed in claim 1, wherein said distal end is rounded so as to facilitate insertion of said fusion cage relative to one or more bone structures.
 3. The fusion cage as claimed in claim 1, wherein said conically-shaped cage body has a conically-shaped outer surface.
 4. The fusion cage as claimed in claim 3, wherein said conically-shaped outer surface of said cage body has continuous threads.
 5. The fusion cage as claimed in claim 3, wherein said conically-shaped outer surface has at least one flute formed therein.
 6. The fusion cage as claimed in claim 5, wherein said flute extends from said distal end toward said proximal end of said cage body.
 7. The fusion cage as claimed in claim 6, wherein said flute is formed in the rounded distal end of said cage body.
 8. The fusion cage as claimed in claim 1, wherein said cage body further comprises: an internal cavity surrounded by said conically-shaped outer surface of said cage body; and a plurality of apertures formed through said conically-shaped body for providing communication between said conically-shaped outer surface and said internal cavity.
 9. The fusion cage as claimed in claim 8, wherein said apertures increase in size from said distal end to said proximal end of said cage body.
 10. The fusion cage as claimed in claim 8, further comprising a bone-growth inducing material disposed in said internal cavity of said cage body.
 11. The fusion cage as claimed in claim 10, wherein said bone-growth inducing material includes bone.
 12. The fusion cage as claimed in claim 10, wherein said proximal end includes an opening in communication with said internal cavity of said cage body so that said bone-growth inducing material may be disposed in said internal cavity of said cage body.
 13. The fusion cage as claimed in claim 12, further comprising an end cap covering said opening for retaining said bone-growth inducing material in said internal cavity.
 14. The fusion cage as claimed in claim 13, wherein said opening at the proximal end of said cage body includes internal threads and said end cap includes external threads adapted for meshing with the internal threads of said opening at said proximal end.
 15. A fusion cage for promoting fusion with one or more bone structures comprising: a conically-shaped cage body including a proximal end having a first diameter and a distal end having a second diameter, wherein said first diameter of said proximal end is greater than said second diameter of said distal end; said cage body defining a conically-shaped outer surface surrounding an internal cavity of said cage body; and a bone-growth inducing material disposed in said internal cavity of said cage body.
 16. The fusion cage as claimed in claim 15, further comprising a plurality of apertures formed through said conically-shaped body for providing communication between said conically-shaped outer surface and said internal cavity.
 17. The fusion cage as claimed in claim 16, wherein said apertures increase in size from said distal end to said proximal end of said cage body.
 18. The fusion cage as claimed in claim 17, wherein said bone-growth inducing material includes bone.
 19. The fusion cage as claimed in claim 15, wherein said proximal end of said cage body includes an opening in communication with said internal cavity of said cage body so that said bone-growth inducing material may be disposed in said internal cavity of said cage body.
 20. The fusion cage as claimed in claim 19, wherein said opening at the proximal end of said cage body includes internal threads and said end cap includes external threads adapted for meshing with the internal threads of said proximal end opening. 